The demand for non-volatile semiconductor memory devices has significantly increased because data stored in the non-volatile memory device may not be erased when power is removed from the device. Flash memory devices may be used as non-volatile memory devices. In these devices, memory cells may be provided by a single transistor. Thus, memory devices including flash memory may be relatively smaller than other memory devices. Therefore, flash memory may be used instead of, for example, magnetic disks, for storing large amounts of data. Details with respect to conventional non-volatile semiconductor memory devices are discussed in U.S. Pat. No. 5,751,634 (the '634 patent) entitled NON-VOLATILE SEMICONDUCTOR MEMORY DEVICE FOR STORING MULTIVALUE DATA AND READOUT/WRITE-IN METHOD THEREFOR to Itoh, the disclosure of which is incorporated herein by reference as if set forth in its entirety.
In particular, the '634 patent discusses a flash memory device including memory cells for storing 2-bit data connected to a bit line. As discussed therein, first and second flip-flop circuits are coupled to the bit line. The first flip-flop circuit stores the lower bit (least significant bit (LSB)) of 2-bit data read out from or written into the memory cell. Similarly, the second flip-flop circuit stores the upper bit (most significant bit (MSB)) of 2-bit data read out from or written into the memory cell. Thus, during a data read operation, the MSB bit is read out of the memory cell first and then the LSB is read out from the memory cell. Similarly, during a data write operation, the MSB bit is written first into the memory cell and then the LSB is written into the memory cell.
Thus, the non-volatile semiconductor memory devices discussed in the '634 patent may provide a memory device having a large capacity, but capable of being manufactured at a low cost without using a difficult patterning technique or developing a new manufacturing technique. However, improved memory devices may still be desired.